As photolithography is pushed to fabricate deep-subwavelength devices, photomask (or mask) preparation is becoming more critical than ever before. This is in part due to the fact that mask patterns have become more complex than conventional Manhattan shapes. Multi-patterning and other techniques needed to extend the 193 immersion capabilities usually depend on a large amount of decoration with optical proximity correction (OPC) shapes. Unlike simple orthogonal SRAFs, the SRAFs for 22 nm/20 nm technology nodes tend to be blobs or curvilinear lines. In the contact and via layers, and particularly for isolated features, extensive use of sub-resolution assist features (SRAFs) is needed to produce the required process window. Another source of complex mask patterns is inverse lithography. Masks computed through use of inverse lithography are known to provide significantly better lithographical performance even than conventional model-based OPC. Such masks, however, generally contain patterns with smaller segments and curved shapes.
Masks that preserve natural output shapes from inverse lithography solutions are shown to have better lithography process window than their Manhattan counterparts. To manufacture masks with complex shapes, a straightforward approach is to represent curvilinear shapes with Manhattan segments. This may not only lead to a large number of shots, but also affect mask pattern fidelity. Model-based mask data preparation (MDP) methods may achieve shot count savings and preserve curved shapes. Some of these methods use smooth and curved contours as mask targets and adjust the numbers, positions and sizes of rectangular shots for optimization. No matter which model-based methods are employed, there will still be residue mask errors. These mask errors can be amplified upon wafer print, degrading pattern image fidelity and resulting in assist feature printing. The sensitivity of the wafer print to mask errors cannot be predicted without information about the optical configuration that will be used to print the wafer. Also the sensitivity is expected to vary so widely across the layout as to preclude the application of a single guard band specification. It is thus desirable to combine mask data preparation with optical proximity correction (OPC), at least in a refinement process.